1. Field of the Invention
The present invention relates to a process of producing a polymerized toner for various recording method, e.g., electrophotography, electrostatic recording and toner-jet recording.
2. Related Background Art
A number of electrophotographic methods are known. In general, they use a photoconductive material to form an electrical latent image on a photosensitive member by various methods, develop the latent image with a toner to make it visible, transfer the toner image to a transfer medium, e.g., paper, as necessary, and fix the toner image on the transfer medium by the aid of heat, pressure or the like to produce the copy or print.
Various methods have been also proposed for developing images by the aid of toner, and fixing toner images.
The conventional processes to produce the toner for the above purposes generally involve uniform dispersion of a colorant, e.g., dye or pigment, in a thermoplastic resin by melting/kneading, finely divide it by a milling machine or the like, and classify the above particles by a classifier to have the toner of desired particle size.
These processes can produce the toner of fairly high quality, but have several limitations, e.g., limitation of the toner materials. For example, the resin dispersed with the colorant should be sufficiently fragile to be finely divided by an economically viable machine system. When the resin is sufficiently fragile to satisfy the above requirements, the fine particles prepared by a high-speed milling machine tend to have a broad size range, into which the fine particles fall at a relatively high proportion. Moreover, the toner of such a fragile material tends to be further divided or powdered in a developing device, e.g., copier. Another problem involved in this process is difficulty in dispersing the fine solid particles (e.g., of colorant) in the resin, which may deteriorate image quality, depending on extent of the dispersion, resulting in, e.g., increased fogging, decreased image concentration, mixed colors, and insufficient transparency. Therefore, care must be fully taken to well disperse the colorant. Moreover, exposure of the colorant to the fractured planes of the milled particles may fluctuate the developing characteristics.
In order to solve the problems involved in the milled toners, various toners and processes of their production have been proposed. For example, Japanese Patent Publication Nos. 36-10231 and 43-10799 disclose various types of polymerized toners, including suspension-polymerized toners, and their production processes.
For example, the suspension polymerization for producing the toner includes preparing a uniformly dispersed or dissolved monomer composition comprising a polymerizable monomer, colorant, polymerization initiator, and, as required, crosslinking agent, charge-controlling agent and one or more other additives; dispersing the monomer composition in a medium containing a dispersion stabilizer, e.g., an aqueous phase, using an adequate agitator; and, at the same time, polymerizing the composition, to produce the toner particles of desired size. This process has various advantages. For example, the toner particles are not necessarily fragile, because no milling step is involved, and soft resin may be used. No colorant is exposed to the particle surfaces, and the toner of uniform friction-induced charging characteristics can be produced. The toner thus produced has a relatively sharp particle size distribution, needing no classification step or classification to only a slight degree, to increase toner yield.
However, the suspension polymerization directly produces the toner in a medium, thus needing a step of separating the toner particles from the medium. Japanese Patent Application Laid-Open No. 8-160661 discloses a polymerized toner and process of producing the same, proposing dehydration and washing steps by a continuous belt filter and siphon type centrifuge. These devices can separate the polymerized toner by filtration. However, they are very large, and, using a filter, involve the problems associated with clogging of the filter. As a result, the through-put may be reduced, when the filter is used repeatedly. No filtration is effected, when the filter is completely clogged, thus a step of washing the filter is needed. It is essential, therefore, to prepare an excess quantity of wash water or spare filter, which pushes up the cost.
Emulsion agglomeration also needs separation of the toner particles from the medium, when used for producing the toner, sharing the common problems with suspension polymerization.
It is an object of the present invention to provide a process of producing polymerized toner which efficiently separates the polymerized toner particles from the medium by filtration.
It is another object of the present invention to provide a process of producing polymerized toner which satisfactorily removes the dispersion stabilizer from the toner particles.
The process of the present invention produces a polymerized toner by polymerizing a polymerizable monomer composition comprising a polymerizable monomer and colorant in a medium to produce the colored resin particles, separating the colored resin particles from the medium by filtration, and drying them, wherein the filtration is effected by a decanter type centrifugal separator equipped with an outer rotary cylinder which contains a screw conveyor freely rotating relative to the cylinder.